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I'm an inexperienced C# hobbyists who tries to learn a bit of c# when I have a little bit of time left, like now lately.

I'm trying to figure out when to use ICollection<T>.

I have searched on YouTube and online documentation. But they cover "how to use it", not why and when to use it. For example a title like below would be far more helpful to me:

"Hey, are you always using List<T>? Find out how ICollection<T> could be more helpful!"

Nothing seems to be out there. I assume one has to learn this in some indirect way. Either way, can anyone provide and answer and even perhaps an example why one would use this, instead of one of lists, arrays and whatnot?

Edit: I don't even know how to ask a question because I don't know what I'm talking about. Here's the code which triggered my interest:

ICollection<ValidationResult> validationResultList = new List<ValidationResult>();

What is happening here?

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    When you use List<T>, you are using ICollection<T> Commented Apr 20, 2023 at 20:52
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    No, you don't have to implement your own collection when using it. You just say "this method returns an ICollection<string>, and which implementation of that interface it will return, you should not care about". That's what an interface does. It doesn't mean you have to implement it in your own type. Stating that your method accepts or returns an ICollection<T> is what's flexible. It allows you to refactor your code to one day return an array instead of a list, and callers won't have to change a thing, and it allows your callers to pass any collection as long as it implements that interface. Commented Apr 20, 2023 at 20:54
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    It's most useful as an input parameter. I don't need to know whether I am being given a List<T> or a T[] in order to reverse the contents. Why would I force callers to always convert their list to an array in order to call my 'Reverse' method? Commented Apr 20, 2023 at 20:56
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    That being said: declaring a local variable as an interface is usually not necessary. In this case it isn't, because it's passed to a method that accepts an ICollection<T>, so an instance of any class that implements that interface would suffice. Commented Apr 20, 2023 at 21:18
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    CodeCaster + AndrewWilliamson thanks. The combined information as well with the answer by @chart solves it for me. Thanks everybody! Now I understand. Commented Apr 20, 2023 at 21:21

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The simple answer is, use ICollection<T> when you need something more abstract than List<T> (or other collection, such as HashSet<T>).

You use the interface because you may not care about receiving List specifically. You care about receiving an object that implements the methods contracted by the interface. You just want to know that when something is passed into your method, and you then call myArgument.neededMethod(), that you know for certain that this method will have been implemented.

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see the little code snippet in my original (edited) post. What is achieved with that code? Thanks.
In your example, you are instantiating a new List<ValidationResult>. The reason you can use ICollection<ValidationResult> as the type is due to polymorphism. If that isn't something you're familiar with, look it up and spend some time trying to understand the concept and it really underpins Object-Oriented Programming (OOP), C# and Java in particular.
In practice, for your example, there is effectively no difference between: ICollection<ValidationResult> validationResultList = new List<ValidationResult>(); and List<ValidationResult> validationResultList = new List<ValidationResult>();
Yes, thank you. I have learned this through comments. This was just the wrong code to be introduced to this generic collection interface. Would it have been in a parameter its use would have been more easily clear to me. Thanks bro.

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